In consumer and industrial applications there is an increasing need for ubiquitous sensors. For many of these applications optical sensing provides the best resolution and selectivity, but optical sensors are usually based on discrete optical elements and therefore costly.
Many sensing applications require measuring the light spectrum. This usually requires a dispersive element (eg a grating) and a detector or detector array, which are combined in a bulky (1-100 cm) and expensive instrument. This is unacceptable for applications that require compact and cheap solutions—e.g. gas sensing for agriculture, monitoring of industrial processes, medical diagnostics. Moreover, there is a trade-off between resolution and size: Making the spectrometer smaller affects its resolution. Attempts at integrating the grating element on an optical chip typically result in a poor resolution (several nm). Additionally, producing large arrays of spectrometers for high-resolution hyperspectral imaging is presently not possible. A second application area is the measurement of mechanical motion at the picometer scale, and correspondingly force and acceleration. While this can be done with optics (e.g. using interferometric methods), it usually requires complex and bulky systems.
Integrated spectrometer implementations are mostly based on arrays of filter elements, which limits the resolution, and rely on external detectors, resulting in a much increased packaging complexity and cost. In principle, the combination of a tuneable optical cavity and a photodetector can lead to an extremely compact spectrometer, particularly if the detector is integrated inside the cavity. However, for many applications high resolution is needed under a wide range of incident angles and over a wide spectral range. This can only be achieved by a wavelength-scale cavity combining low optical loss, wide free spectral range (FSR) and large tuneability. So far, tuneable microcavity detectors have achieved limited resolution and spectral range.
What is needed is an optical sensor, having outstanding resolution and bandwidth, which is fully integrated and mass-manufacturable.